My 92 F-150 failed emissions/smog test today . The hydrocarbons are too high, and its running a little rich. The truck came with a 302 but it has a 351 now. It has passed the test before with the 5.8 so I don’t think that's the issue. I have fairly new cats, about 3 years old, so I don’t think that’s the problem either. Here is my best guess. The temp gauge never hardly gets even into the cool side of the normal operating range. It does come up a little but not more than even a quarter of the way up. I have good heat, real good heat actually, so I know its circulating. I changed out the thermostat because I happened to have one sitting around and that didn’t make any difference. I'm guessing maybe its the temp sensor? Here is my question. If the temp sensor is thinking that the motor is still cold wont that make the truck run rich? Also, will the truck still run rich even if it is warming up but the sensor is bad and thinks it’s still cold? Do you think this could be causing it to not pass emissions?

 

Keep in mind that there are two temp sensors, one for the gauge and one for the computer. If you're getting good heat, I'm disinclined to blame the thermostat. The gauge info has nothing to do with the temp reported to the computer. Are you getting a check engine light?

 

No check engine light. And I found that the sensor that is by the distributer is not even hooked up. It is taped over and I dont even see any wires to plug it in. The other sensor thats facing the front of the motor is still plugged in

 

ah the joys of living in California, we don't even have emission testing in Wyoming.

 

If your hydrocarbens are high that is useualy an ignition or compression problem causing a missfire a rich mixture will usualy cause hi CO. reading. check your plugs.wires and the cap and roter.

 

Even though the light isn't lit up, check for codes anyway. With it being emissions related, 99% of the time there will be a code. You may be lucky enough to be in the 1%, though. I would start with checking your ignition and O2 sensors. Even though your cats are only 3 years old, if it's been running rich long enough, your cats could be dead. The ECT sensor for the computer I BELIEVE is by the distributor. The other for the dash I think is in the drivers side cylinder head by the spark plugs.

 

the temp sensor can cause a rich mixture but the O2 is supposed to pull it back...

check the voltages of both and that will tell you if one is triggering the other...

Make sure you have a new cap and rotor, plugs and wires and while you're at it replace teh Ignition Coil with a new one... a coil gets weak and that will cause the rich mixture over the other 2 things....

 

Simple fix. Fill tank half way, go to the home depot, buy a quart of denatured alcohol, pour in tank, drive around until they mix, idle might be a little off, it worked for me a bunch of times.

 

Why go to all the trouble just put in a gallen of E85 same thing

 

I went back and looked at the test results and it failed because of the hydorcarbons, but it is showing elevated levles of CO but not to the point that it failed because of them. Guess a good tune up is in order first. I suppose I will start a new thread to discuess my transmission problems next!